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Fault clearance time is the time from fault current to when fault current stops.At the moment of a fault the fuse is just a wire so the supply will deliver the maximum current that can be drawn from it.At this point the fuse element will begin to heat based upon volt drop and current drawn.When the element breakes, the gases and arc continue to deliver fault current until the gap/arc can no longer sustain conduction.NB this clearence is different between AC and DC supplies, where the AC voltage changes and reduces arc distance.On DC the votage remains constant so the arc is sustained and more difficult to break, also any capacitors or storage can deliver high current pulseswhere the energy dissipated in the fault is based upon discharge times and current/voltage. Where the voltage can sustain the arc.On high energy disruption it is possible for a cartridge fuse to rupture or explode due to expanding gasses and heating effect.Some designs use fuse elements under tension which pull apart as the fuse operates, others are filled with silica/sand which falls into the gap in the element during operation to help quench the arc.

On a cartridge fuse, fusing current is 1.5 times the rated value.Some fuses are rated for slow blow where a large inrush or magnetisation current happens at switch on eg toroid Tx.This would blow a fuse at power on.Quick blow is used where fast action is required for electronics where a surge would cause significant damage but no inrush surge takes place.Fuses have a voltage rating!

Hi, Yes a good idea to keep in mind. If you use a 100 amp one and it will never blow. For tube gear I recommend a slow blow rated at about 2 times the expected current. For normal fuses I use 3-4 times the expected current. Any less and you will be replacing fuses often when there is no failure. My experience is that any significant failure in tube gear especially power amps will easily blow a fuse many times the ratings I indicated. Fortunately tube gear itself is rather hardy and need not have instaneous fuse action. Some SS gear however is not so tolerant.

On most projects, but I think they are a "must fit" with toroid transformers.It also means you can fuse closer to the running current so you don't have to fit massively over rated fuses.I do use polyswitch solid state fuses on heater circuits.

Some people use microwave fuses for high voltage clearence.

I use capacitor quench across none safety critical fuses it stops them exploding on HV.The sound of a fuse<<lets not go there.

I think its interesting for newbies to realise the energy that goes into a fault at the instant it happens before a fuse blows and that the clearance time limits the energy into the fault and heating effect.But remember the fuse is just a "piece of wire" so it has no magical properties at the moment of short circuit.

They need to do a "Readers Digest" version for those of us with low attention sp... SQUIRREL!

Yes I know fuses are dead boring So I thought a quick thread would cut through the fact that many people just fit them but don't actually know how they work or how to rate them.Things like voltage rating gets lost with many thinking there is only a current rating.Also the effect of DC Vs AC on fusing and clearance times.

Things like voltage rating gets lost with many thinking there is only a current rating.Also the effect of DC Vs AC on fusing and clearance times.

Too true!

Arc quenching should also be a consideration with large transformers - those sand filled or "ceramic microwave oven" fuses as they're commonly known.

I had a Citation PA in that blew fuses and a regular one went *ssssst* as it blew, with a bright light that shone out of the holder. Orig. spec was the spring-loaded ones, but I had sand-filled ones on hand. Blew one more before tracking the issue, but no *ssssst*, just opened normally

So no the fuse won't stop the fault happening.Its not meant to protect the equipment just prevent a fire and damage to wires and cables.If a component fails or insulation break down then that's it component is no longer serviceable.The fuse is just damage limitation. Its not a current limiter.

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